1. Field of the Invention
The present invention relates to an image processing apparatus which processes a still image produced by an image pickup device or a still image area in a motion picture.
2. Related Background Art
An image processing apparatus which applies various processing to image data produced by an image pickup unit and outputting it has been known. In this type of apparatus, as shown in FIGS. 3A and 3B, in order to increase an object field depth of the image pickup unit 101, a diaphragm 102 is stopped down from a proper stop value shown in FIG. 3A to a position shown in FIG. 3B. Usually, since a level of an image signal of the image pickup unit 101 is lowered by the stop-down, an illumination of a light source 103 for illuminating the object as shown in FIG. 3B is increased by an amount corresponding to the reduction of the level of the image signal.
However, as the brightness of the illumination light source is increased, it may adversely affect to the object. Further, where a natural light is used as the illumination light source, it is not possible to increase the illumination.
In another type of image processing apparatus, image signals at different time points correlated to each other are circulatively summed to reduce a noise. This is referred to as a circulation type noise reduction apparatus.
A prior art apparatus of this type is configured as shown in FIG. 10, in which a video signal applied to an input terminal 1 is multiplied by a multiplier 2 by a factor of K, where K is 0.ltoreq.K.ltoreq.1. An output of the multiplier 2 is outputted through an adder 3 and it is also supplied to a unit delay element 4. An output of the unit delay element 4 is delayed by one frame by a frame memory, multiplied by a multiplier 5 by a factor of (1-K), and supplied to the adder 3 where it is summed with the signal from the input terminal 1. In general, since noises of the same pixel are not correlated between frames, the noise component of the video signal is reduced by repeating the above operation.
In the prior art described above, since the factor K is fixedly set in accordance with the amount of noise component included in the video signal, the following problem arises.
Assuming that K=1/2, the number of times of circulative addition and the degree of reduction of noise are considered. In an initial state (which is defined as a first circulative addition), the video signal applied to the input terminal 1 is produced from the output terminal 6 as it is. In a second addition by the adder 3, 1/2 time of the input video signal and 1/2 time of the video signal of the same pixel of one frame older are summed. An amplitude component of the noise in the output of the adder 3 is given by ##EQU1## where N is an amplitude component of the noise in the video signal. The first term in represents a noise power of the output of the multiplier 2, and the second term represents a noise power of the output of the multiplier 5. Since K=1/2, ##EQU2## Namely, the noise is reduced by the factor of 1/.sqroot.2. Hereinafter, the value corresponding to 1/.sqroot.2 is defined as a noise reduction factor.
The noise reduction factors when K=1/2 are sequentially calculated as follows: ##EQU3##
The result of calculation for K=1/2 is graphed as shown in FIG. 6. Similar calculations were made for K=2/3 and K=1/10 and the results are shown in FIG. 6.
As seen from FIG. 6, when K is large, the convergence of the noise reduction factor is fast but a convergence value is large and the noise reduction effect is small. On the other hand, when K is small, the convergence value of the noise reduction factor is small, but the convergence is slow. Thus, in the prior art noise reduction apparatus, it is not possible to quickly and sufficiently reduce the noise of the still image.